Purpose: While auto tube current modulation is an effective dose reduction technique, manufacturer-specific implementation of the technique leads to inconsistencies between scans obtained on different scanners. This research describes efforts to translate comprehensive scan protocols from a reference CT scanner to progeny scanners by matching x-ray output using a variable diameter phantom.

Methods: The phantom consisted of six cylinders of PMMA with increasingly large diameters, much like a wedding cake. The diameters of the cylinders were 10, 13, 16, 20, 25, and 32 cm. The reference CT scanner was a GE LightSpeed 16; the progeny scanners were a GE LightSpeed VCT and Siemens Definition AS+. The phantom was scanned while varying the tube modulation metric (either noise index or reference mAs). Using custom software, the mAs was determined on a slice-by-slice basis and then averaged along the length of each phantom. Two conversion metrics were examined as a way to translate x-ray output between scanners: (1) CTDIw or (2) air exposure. The modulation metric of the progeny CT scanner that produced the output closest to that of the reference CT scanner was then selected as the optimal metric for each phantom size. The metric was then compared to the ad hoc scan protocols currently used for the progeny scanners.

Results: The range of appropriate tube modulation metrics is highly dependent on patient diameter. When applied to patients of varying sizes, tube current modulation may be hitting the maximum or minimum limits.

Conclusions: Both CTDIw and air exposure are reasonable conversion metrics to match x-ray output between CT scanners. While matching x-ray output is the first step towards standardizing scan protocols across different types of scanners, thorough image quality analysis must be done to ensure that clinical tasks can be performed sufficiently.